1. Field of the Invention
This invention relates to a dry system microscope objective lens of medium or high magnification.
2. Description of the Prior Art
Heretofore, in microscope objective lenses of medium or high magnification, the working distance has generally been small and for example, in an objective lens of the order of forty times, the working distance has been 0.2-0.5 mm, and in an objective lens of the order of one hundred times, the working distance has been 0.1-0.3 mm. In objective lenses of such small working distances, the fore end thereof has been liable to touch an object to be examined during the operation of the microscope to injure the object to be examined and also, an inconvenient situation has been liable to occur during operation. Therefore, objective lenses having a high magnification and yet having a great working distance have been desired, but maintenance of the planarity of the image plane and correction of aberrations have been more difficult as the magnification becomes higher.
Also, generally, microscope objective lenses used under transmission illumination like those for biological purposes are designed on the premise that the thickness of the cover glass is a predetermined reference value and therefore, where the thickness of the cover glass differs from the reference value, the imaging performance of the objective lens is deteriorated. Such tendency becomes more remarkable as the N.A. (numerical aperture) of the objective lens is greater. For this reason, as an objective lens with a correction ring, there is known an objective lens in which the lens spacing in the objective lens is varied with a variation in the thickness of the cover glass to thereby prevent aggravation of aberrations and maintain a substantially good imaging performance. However, in the conventional popular objective lens with a correction ring, the range of aberration correction for the variation in the thickness of the cover glass is very narrow, and in the case of N.A. of the order of 0.6, the thickness range of 0.2-0.3 mm has been the practical limit.
In contrast, Japanese Laid-open Patent Application No. 142508/1981 (corresponding U.S. Pat. No. 4,403,835) discloses a technique in which a microscope objective lens comprises, in succession from the object side, a first lens group which is a positive cemented meniscus lens having its concave surface facing the object side, a second lens group which is a positive lens or a cemented positive lens and a third lens group of positive synthesized refractive power and wherein only the second lens group is moved along the optical axis in accordance with a variation in the thickness of a parallel flat plate disposed between the object surface and the objective lens, whereby a good imaging performance is maintained even if there is a wide range of variation in the thickness of the parallel flat plate. According to this technique, it is certainly possible to maintain an excellent imaging performance over a very wide range of variation in the thickness of the parallel flat plate, say, .+-.1.0 mm. In this technique, however, an objective lens having N.A. of the order of 0.6 and a magnification of the order of 40 times is the practical limit, and this has been insufficient as an objective lens having a greater N.A. or a higher magnification.